Avoid "could not reparent node" error when removing every tag.

This is a fix for issue [#50](https://github.com/cantino/ruby-readability/issues/50).

When trying to remove all tags from the doc, the "could not reparent node" is thrown from Nokogiri. That's because we are reparenting a root node. Some logic is added to check if it is root to avoid the error.